The unfolded protein response (UPR) is a stress response activated upon disturbed homeostasis in the endoplasmic reticulum (ER). Previously, we reported that the activation of the UPR closely correlates with the presence of phosphorylated tau (p-tau) in Alzheimer's disease (AD). As well as increased presence of intracellular p-tau, AD brains are characterized by extracellular deposits of β amyloid (Aβ). Recent in vitro studies have shown that Aβ can induce ER stress and activation of the UPR. The aim of the present study is to investigate UPR activation in sporadic tauopathies like progressive supranuclear palsy (PSP) and Pick's disease (PiD), and familial cases with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) which carry mutations in the gene encoding for tau (MAPT ). The presence of phosphorylated pancreatic ER kinase (pPERK) and phosphorylated inositol requiring enzyme 1α (pIRE1), which are indicative of an activated UPR, was assessed by immunohistochemistry in cases neuropathologically defined as frontotemporal lobar degeneration with tau pathology (FTLD-tau). Increased presence of UPR activation markers pPERK and pIRE1 was observed in neurons and glia in FTLD-tau cases, in contrast to FTLD subtypes negative for tau pathology or in non-neurological controls. pPERK and pIRE1 were also prominently present in relatively young carriers of MAPT mutation. A strong association between the presence of UPR activation markers and p-tau was observed in the hippocampus of FTLD-tau cases. Double immunohistochemical staining on FTLD-tau cases revealed that UPR activation is predominantly observed in neurons that show diffuse staining of p-tau. These data demonstrate that UPR activation is intimately connected with the accumulation and aggregation of p-tau, and occurs independently from A? deposits. Our findings provide new pathological insight into the close association between p-tau and UPR activation in tauopathies. Copyright © 2012 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.